Researchers tested the brains of 48 volunteers, half of whom were given weekly juggling lessons for six weeks. At the end of that period, their brains were tested again. Those who had juggled—regardless of whether they had become good jugglers—had a 5 percent increase in their brains’ white matter, the nerve strands that connect different parts of the brain. Those who hadn’t juggled had little change.

“Knowing that pathways in the brain can be enhanced may be significant in the long run in coming up with new treatments for neurological diseases, such as multiple sclerosis, where these pathways become degraded,” said Johansen-Berg.

“The brain is like a muscle, we need to exercise it,” said Dr. Arne May, lead researcher of the German study.

There have been studies showing that games and puzzles such as Brain Age and Sudoku keep brains healthy and improve mental skills. Several studies have had promising results giving brain games to the elderly, people with forms of dementia and children with ADHD.

However, the findings on brain training can be misleading, according to Dr. Adrian Owen of the BBC’s Lab UK, who writes that many studies aren’t peer-reviewed, and don’t have a proper control group. He also says that brain imaging, which was used in the Oxford and German juggling studies, “only shows how hard the brain is working, rather than how effective the training is.”

Lab UK is organizing an experiment, called “Brain Test Britain,” that allows Internet users to take online brainteaser tests, use brain training games for at least 6 weeks, and then take the tests again to see if there is an improvement. The study will run for the next year and be published in 2011.